Objection 1. It would seem that prudence is not a virtue. For Augustine says (De Lib. Arb. i, 13) that "prudence is the science of what to desire and what to avoid." Now science is condivided with virtue, as appears in the Predicaments (vi). Therefore prudence is not a virtue.
Objection 2. Further, there is no virtue of a virtue: but "there is a virtue of art," as the Philosopher states (Ethic. vi, 5): wherefore art is not a virtue. Now there is prudence in art, for it is written (2 Chronicles 2:14) concerning Hiram, that he knew "to grave all sort of graving, and to devise ingeniously [prudenter] all that there may be need of in the work." Therefore prudence is not a virtue.
Objection 3. Further, no virtue can be immoderate. But prudence is immoderate, else it would be useless to say (Proverbs 23:4): "Set bounds to thy prudence." Therefore prudence is not a virtue.
On the contrary, Gregory states (Moral. ii, 49) that prudence, temperance, fortitude and justice are four virtues.
I answer that, As stated above (I-II, 55, 3; I-II, 56, 1) when we were treating of virtues in general, "virtue is that which makes its possessor good, and his work good likewise." Now good may be understood in a twofold sense: first, materially, for the thing that is good, secondly, formally, under the aspect of good. Good, under the aspect of good, is the object of the appetitive power. Hence if any habits rectify the consideration of reason, without regarding the rectitude of the appetite, they have less of the nature of a virtue since they direct man to good materially, that is to say, to the thing which is good, but without considering it under the aspect of good. On the other hand those virtues which regard the rectitude of the appetite, have more of the nature of virtue, because they consider the good not only materially, but also formally, in other words, they consider that which is good under the aspect of good.
Now it belongs to prudence, as stated above (1, ad 3; 3) to apply right reason to action, and this is not done without a right appetite. Hence prudence has the nature of virtue not only as the other intellectual virtues have it, but also as the moral virtues have it, among which virtues it is enumerated.
Reply to Objection 1. Augustine there takes science in the broad sense for any kind of right reason.
Reply to Objection 2. The Philosopher says that there is a virtue of art, because art does not require rectitude of the appetite; wherefore in order that a man may make right use of his art, he needs to have a virtue which will rectify his appetite. Prudence however has nothing to do with the matter of art, because art is both directed to a particular end, and has fixed means of obtaining that end. And yet, by a kind of comparison, a man may be said to act prudently in matters of art. Moreover in certain arts, on account of the uncertainty of the means for obtaining the end, there is need for counsel, as for instance in the arts of medicine and navigation, as stated in Ethic. iii, 3.
Reply to Objection 3. This saying of the wise man does not mean that prudence itself should be moderate, but that moderation must be imposed on other things according to prudence.
Source: Summa Theologica (New Advent)